This invention relates to metal-containing dendrimers and light-emitting devices containing them.
A wide range of luminescent low molecular weight metal complexes are known and have been demonstrated as both light emitting and charge transporting materials in organic light emitting devices, in particular light emitting diodes (LEDs) also known as electroluminscent (EL) devices. Analysis of spin statistics associated with the injection of oppositely charged carriers which pair to form excitons shows that only 25% of the excitons formed in the LED are in the singlet state. Although it has been suggested that the barrier of 25% for singlet excitons may be exceeded under certain circumstances it is known to be far from 100%. For most organic materials only the singlet states can decay radiatively generating light, the triplet states decay non-radiatively. The possibility to extract luminescence from the triplet excited state has recently been demonstrated by inclusion of phosphorescent guest metallic complexes in host matrices. However, blend systems are sensitive to the concentration of the guest in the host and only low concentrations of the guest can be used before phase separation leads to aggregation and quenching.
In addition the metal complexes used to date have been designed to be volatile so that layers can be deposited by thermal evaporation. In many applications solution processing would be preferable to evaporation, but the current materials do not form good films when deposited by solution processing. In addition it would be advantageous to have guest host systems in which high levels of guest can be used. This is possible with dendritic materials.
According to the present invention these problems are solved by forming certain dendrimers with metal ions as part of the core. Dendrimers are highly branched macromolecules in which branched dendrons (also called dendrites) are attached to a core. The properties of the dendrimers make them ideal for solution processing and allow incorporation of metal complex chromophores, which have been demonstrated to be effective in light emitting devices (LEDs), into a solution processable system.
The known examples of metal containing dendrimers fall into three classes                (i) metal ion at the centre        (ii) metal ions on the periphery        (iii) metal ions at the branching points        
There is a range of metal containing dendrimers which have metal ions as part of the branching points and coordinating groups linking the metal ions, (see, e.g., Chem. Comm. (2000) 1701 and Adv. Mater. 10(4) (1998) 295). The photoluminescent properties of some of these materials have been studied in solution, but the solid state luminescent properties have generally not been explored. It does not necessarily follow that a material that is luminescent in solution will also be luminescent in the solid state. Concentration quenching in the solid state is a common occurrence. In the dendrimers with metals ions at the branching points there is a high density of chromophores which makes concentration quenching particularly likely. Similarly in dendrimers with metal ions at the periphery, the metal ions in adjacent molecules will be close and again concentration quenching will be a problem.
The current invention is directed towards dendrimers with metal ions as part of the core. When the metal ion chromophore is sited at the core of the molecule, it will be relatively isolated from the core chromophores of adjacent molecules, which minimizes possible concentration quenching or triplet-triplet annihilation.
Further the organometallic dendrimers already disclosed generally do not have conjugated dendrons, and so are unlikely to work well in an electroluminscent device. For example the only lanthanide (Ln) dendrimers reported to date have a Ln core and benzyl ether Frechet-type dendrons. These compounds were shown to give PL emission, but have not been proven in EL devices. Kawa, M.; Frechet, J. M. J. Thin Solid Films, 331 (1998) 259]
Some organic dendrimers have been demonstrated to work in organic light emitting devices. However, the use of metal ion chromophores in the dendrimers opens up the range of materials that can be used, and may offer benefits in terms of stability and/or charge transport compared to organic systems. A particular benefit is the potential for highly efficient solution processed phosphorescent systems. Accordingly, the present invention provides a light emitting device which comprises a layer containing a metal ion containing dendrimer and, in particular, an organometallic dendrimer with a metal cation as part of its core, said core(o centre) not comprising a magnesium chelated porphyrin.
The present invention is particularly directed towards the use of dendrimers containing one or more at least partially conjugated organic dendrons with a metal ion as part of the corel. Such dendrimers form another aspect of the present invention. The atoms or groups coordinating/binding to the metal typically form part of the core itself e.g. fac-tris (2-phenylpyridyl) iridum III. Thus the dendrirners typically have the formula (I):CORE-[DENDRITE]n  (I)in which CORE represents a metal ion or a group containing a metal ion, n represents an integer of 1 or more, each DENDRITE, which may be the same or different, represents an inherently at least partially conjugated dendritic structure comprising aryl and/or heteroaryl groups or nitrogen and, optionally, vinyl or acetylenyl groups connected via sp2 or sp hybridised carbon atoms of said (hetero)aryl vinyl and acetylenyl groups or via single bonds between N and (hetero)aryl groups, CORE terminating in the single bond which is connected to an sp2 hybridised (ring) carbon atom of the first (hetero)aryl group or single bond to nitrogen to which more than one at least partly conjugated dendritic branch is attached, said ring carbon or nitrogen atom forming part of said DENDRITE. It is to be understood that the term “metal ion” or “metal cation”, as used herein, describes the charge state the metal would have without any ligands attached (the oxidation state). In the dendrimers that contain a metal cation the overall charge of the dendrimer is neutral and the metal-ligand bonding will have more or less covalent character depending on the metal and ligand involved.
As used herein the term acetylenyl refers to acetylenyl groups that are di-valent, vinyl refers to vinyl groups that are di- or tri-valent, and aryl refers to aryl groups that are di-, tri- or multivalent. In a preferred embodiment the dendrites are conjugated.
The dendrimers of the invention are preferably luminescent in the solid state. The luminescent moiety may be partially or wholly within the core itself. The luminescence is preferably from the metal complex.
More preferably the organometallic dendrimer of the invention is phosphorescent in the solid state.
Suitable branching points include aryl and heteroaryl, which can be fused, aromatic ring systems and N. The links between branching points include bonding combinations such as aryl-aryl, aryl-vinyl-aryl, aryl-acetylenyl-aryl, aryl-aryl′-aryl (where aryl′ may be different from aryl), N-aryl and N-aryl′-N. An individual dendron may contain one or more of each type of branching point. Moreover, in the case of the aryl-vinyl-aryl and aryl-acetylenyl-aryl linkages within the dendron there may be one or more aryl- vinyl or aryl-acetylenyl link between the branching points. Indeed there may be more than one vinyl or acetylenyl or aryl moiety between two aryl groups but preferably no more than three. Further, there can be advantages in using an asymmetric dendrimer i.e. where the dendrons are not all the same.
Thus the dendrimers may be ones having the formula (II):CORE-[DENDRITE1]n[DENDRITE2]m  (II)in which CORE represents a metal ion or a group containing a metal ion, n and m, which may he the same or different, each represent an integer of at least 1, each DENDRITE1, which may be the same or different when n is greater than 1, and each DENDRITE2, which may be the same or different when m is greater than 1, represent dendritic structures, at least one of said structures being fully conjugated and comprising aryl and/or heteroaryl groups or nitrogen and, optionally, vinyl and/or acetylenyl groups, connected via sp2 or sp hybridized carbon atoms of said (hetero)aryl, vinyl and acetylenyl groups or via single bonds between N and (hetero)aryl groups, and the branching points and/or the links between the branching points in DENDRITE1 being different from those in DENDRITE2, CORE terminating in the single bond which is connected to a sp2 hybridized (ring) carbon atom of the first (hetero)aryl group or single bond to nitrogen to which more than one conjugated dendritic branch is attached, said ring carbon atom or nitrogen forming part of said fully conjugated DENDRITE1 or DENDRITE2 and CORE terminating at the single bond to the first branching point for the other of said DENDRITE1 or DENDRITE2, at least one of the CORE, DENDRITE1 and DENDRITE2 being luminescent, as well as a light emitting dendrimer having the formula (HI):CORE-[DENDRITE]n   (III)in which CORE represents a metal ion or a group containing a metal ion, n represents an integer of 1 or more, each DENDRITE, which may be the same or different, represents an inherently at least partially conjugated dendritic molecular structure which comprises aryl and/or heteroaryl or N and, optionally, vinyl and/or acetylenyl groups connected via sp2 or sp hybridized carbon atoms of said (hetero)aryl, vinyl and acetylenyl groups or via single bonds between N and (hetero)aryl groups, and wherein the links between adjacent branching points in said DENDRITE are not all the same, CORE terminating in the single bond which is connected to a sp2 hybridized (ring) carbon atom of the first (hetero)aryl group or N to which more than one dendritic branch is attached, said ring carbon atom or N forming part of said DENDRITE, the CORE and/or DENDRITE being luminescent. In one aspect of the invention DENDRITE, DENDRITE1 and/or DENDRITE2 does not include N as a branching point and is conjugated.
It is to be understood that in formulae I, II and III CORE does not comprise a magnesium chelated porphyrin.
In this context, conjugated dendrons (dendrites) indicate that they are made up of alternating double and single bonds, apart from the surface groups. However this does not mean that the TC system is fully delocalised. The delocalisation of the TC system is dependent on the regiochemistry of the attachments. In a conjugated dendron any branching nitrogen will be attached to 3 aryl groups.
Preferably in the organometallic dendrimer according to the invention the dendrimer has at least one inherently at least partially conjugated dendron. More preferably the dendrimer has at least two inherently at least partially conjugated dendrons. Most preferably all the dendrons are inherently at least partially conjugated.
The dendrimer may have more than one luminescent moiety. In a preferred embodiment the dendrimer incorporates at least two inherently luminescent moieties which moieties may or may not be conjugated with each other, wherein the dendron includes at least one of the said luminescent moieties. Preferably the luminescent moiety or moieties further from the core of the dendrimer have a larger HOMO-LUMO energy gap than the luminescent moiety or moieties closer to or partly or wholly within the core of the dendrimer. In another embodiment the HOMO-LUMO energy gap is substantially the same although the surface groups may change the HOMO-LUMO energy gap of the chromophores at the surface of the dendrite. Sometimes in, say, the second generation dendrimer the surface group makes the chromophore at the distal end of the dendrite of lower HOMO-LUMO energy compared to that of the next one in.
The relative HOMO-LUMO energy gaps of the moieties can be measured by methods known per se using a UV-visible spectrophotometer. One of the luminescent moieties may be, or be (partly or wholly) within, the core itself, which will thus preferably have a smaller inherent HOMO-LUMO gap energy than the other luminescent moiety or moieties in the dendron. Alternatively, or in addition, the dendrons themselves may each contain more than one luminescent moiety, in which case those further from the core will in preferably have larger inherent HOMO-LUMO gap energies than those closer to the core. In this case, the core itself need not be luminescent, although luminescent cores are generally preferred.
Preferably in an organometallic dendrimer according to the invention the HOMO-LUMO energy gap of the core is lower than that of the conjugated moieties in the dendrons.
Suitable surface groups for the dendrimers include branched and unbranched alkyl, especially t-butyl, branched and unbranched alkoxy, for example 2-ethylhexyloxy, hydroxy, alkylsilane, carboxy, carbalkoxy, and vinyl. A more comprehensive list includes a further-reactable alkene, (meth)acrylate, sulphur-containing, or silicon-containing group; a sulphonyl group; polyether group; a C1-to-C15, alkyl (preferably t-butyl) group; an amine group; a mono-, di- or tri-C1-to-C15 alkyl amine group; a —COOR group wherein R is hydrogen or C1-to-C15 alkyl; an —OR group wherein R is hydrogen, aryl, or C1-to-C15 alkyl or alkenyl; an —O2SR group wherein R is C1-to-C15 alkyl or alkenyl; an —SR group wherein R is aryl, or C1-to-C15 alkyl or alkenyl; an —SiR3 group wherein the R groups are the same or different and are hydrogen, C1-to-C15 alkyl or alkenyl, or an —SR′ group (R′ is aryl or C1-to-C15 alkyl or alkenyl), aryl, or heteroaryl. Typically t-butyl and alkoxy groups are used. Different surface groups may be present on different dendrons or different distal groups of a dendron. It is preferred that the dendrimer is solution processable i.e. the surface groups are such that the dendrimer can be dissolved in a solvent.
The surface group can be chosen such that the dendrimer can be photopatterned. For example a cross-linkable group is present which can be cross-linked upon irradiation or by chemical reaction. Alternatively the surface group comprises a protecting group which can be removed to leave a group which can be cross-linked. In general, the surface groups are selected so the dendrimers are soluble in solvents suitable for solution processing.
The aryl groups within the dendrons can be typically benzene, napthalene, biphenyl (in which case an aryl group is present in the link between adjacent branching points) anthracene, fluorene, pyridine, oxadiazole, triazole, triazine, thiophene and where appropriate substituted variations. These groups may optionally be substituted, typically by C1 to C15 alkyl or alkoxy groups. The aryl groups at the branching points are preferably benzene rings, preferably coupled at ring positions 1, 3 and 5, pyridyl or triazinyl rings. The dendrons themselves can contain a, or the, fluorescent chromophore.
It is possible to control the electron affinity of the dendrimers by the addition to the chromophores of electron-withdrawing groups, where appropriate, for example cyano and sulfone which are strongly electron-withdrawing and optically transparent in the spectral region we are interested in. Further details of this and other modifications of the dendrimers can be found in WO99/21935 to which reference should be made.
It will be appreciated that one or more of the dendrons attached to the core (provided that at least one dendron is a specified conjugated dendron) can be unconjugated. Typically such dendrons include ether-type aryl dendrons, for example where benzene rings are connected via a methyleneoxy link. It will also be appreciated that when there is more than one dendron, the dendrons can be of the same or different generation (generation level is determined by the number of sets of branching points). It may be advantageous for at least one dendron to be of the second, or higher, generation to provide the required solution processing properties.
The cores typically comprise a metal cation and attached ligands; the metal is typically central in the core and the core is typically luminescent. If it is not luminescent one or more of the dendrons should contain a luminescent group.
When the core comprises a metal cation and attached ligands it is typically a complex of a metal cation and one, two or more coordinating groups, at least one, and preferably at least two, of the coordinating groups being bound to a dendron. Typically the luminescence of the dendrimer will derive from that complex. When CORE in formula (I), (II) or (III) above represents a group containing a metal cation, CORE is typically a complex of a metal cation and two or more coordinating groups, at least one and preferably two or more of the said groups each being bound to a DENDRITE, DENDRITE1 or DENDRITE2 moiety as defined in formulae (I), (II) or (III), respectively, by the single bond in which CORE in these formulae terminates.
In one aspect of the invention CORE may be represented as a complex of the following formula (IV):M [X—]qYr   (IV)wherein M is a metal cation, each [X—], which are the same or different, is a coordinating group X attached to a single bond in which CORE terminates, each Y, which may be the same or different, is a coordinating group, q is an integer and r is 0 or an integer, the sum of (a.q)+(b.r) being equal to the number of coordination sites available on M, wherein a is the number of coordination sites on [X—] and b is the number of coordination sites on Y.
The single bond in the, or each, [X—] moiety, being a bond in which CORE terminates, connects to a dendron. Preferably there are at least two dendrons in a dendrimer, in which case q in formula (IV) is an integer of 2 or more. The said two or more dendrons typically have the structures represented by DENDRITE, DENDRITE1 and/or DENDRITE2 as defined in formulae (I) to (III) above. The coordinating groups Y, when present, are neutral or charged chelated ligands which are not attached to dendrons and which serve to fulfil the coordination requirements of the metal cation. Suitable metals include:
lanthanide metals: such as cerium, samarium, europium, terbium, dysprosium, thulium, erbium and neodymium,
d-block metals, especially those in rows 2 and 3 i.e. elements 39 to 48 and 72 to 80: such as iridium, platinum, rhodium, osmium, ruthenium, rhenium, scandium, chromium, manganese, iron, cobalt, nickel and copper, and main group metals of the Periodic Table: such as metals from Groups 1A, IIA, IIB, IIIB e.g. lithium, beryllium, magnesium, zinc, aluminum, gallium and indium. Suitable substituents Y, for rhenium in particular, include CO and halogen such as chlorine. For iridium dendrimers, the part of the ligands attached to the metal is preferably a nitrogen-containing heteroaryl, for example pyridine, attached to a (hetero) aryl where aryl can be a fused ring system, for example substituted or unsubstituted phenyl or benzothiophene. It should also be noted that the pyridine can also be substituted. Platinum dendrimers and especially platinum dendrimers with a porphyrin core with stilbene-based dendrons attached in the meso position are generally less preferred.
Preferably in an organometallic dendrimer according to the invention the metals as part of the core is not magnesium. Preferably in an oranometallic dendrimer according to the invention the metal ions are exclusively at the centre.
It will be appreciated that the light emission can be either fluorescent or phosphorescent depending on the choice of metal and coordinating groups.
Suitable coordinating groups for the f-block metals include oxygen or nitrogen donor systems such as carboxylic acids, 1,3-diketonates, hydroxy carboxylic acids, Schiff bases including acyl phenols and iminoacyl groups. As is known, luminescent lanthanide metal complexes require sensitizing group(s) which have the triplet excited energy level higher than the first excited state of the metal ion. Emission is from an f-f transition of the metal and so the emission colour is determined by the choice of the metal. The sharp emission is generally narrow, resulting in a pure colour emission useful for display applications. Due to the ability to harvest triplet excitons i.e. phosphorescence, the potential device efficiency can be higher than for fluorescent systems.
Main group metal complexes show ligand based, or charge transfer emission.
The emission colour is determined by the choice of ligand as well as the metal. A wide range of luminescent low molecular weight metal complexes are known and have been demonstrated in organic light emitting devices [see, e.g., Macromol. Sym. 125 (1997) 1-48, U.S. Pat. Nos. 5,150,006, 6,083,634 and 5,432,014]. Suitable ligands for di or trivalent metals are shown in FIG. 1; they include oxinoids (I) e.g. with oxygen-nitrogen or oxygen-oxygen donating atoms, generally a ring nitrogen atom with a substituent oxygen atom, or a substituent nitrogen atom or oxygen atom with a substituent oxygen atom such as 8-hydroxyquinolate (IA) and hydroxyquinoxalinol (I B), 10-hydroxybenzo(h)quinolinato (II), benzazoles (III), schiff bases (V), azoindoles (IV), chromone derivatives (VI), 3-hydroxyflavone (VII), and carboxylic acids such as salicylato (VIII) amino carboxylates (IX) and ester carboxylates (X). The substituents including the R and X groups are typically halogen, alkyl, alkoxy, haloalkyl, cyano, amino, amido, sulfonyl, carbonyl, aryl or heteroaryl on the (hetero)aromatic rings which may modify the emission colour. The R groups in formulae V and X are typically alkyl or aryl. The alkyl groups are typically alkyl groups of 1 to 6 carbon atoms, especially 1 to 4 carbon atoms such as methyl, ethyl, propyl and butyl. The aryl groups are typically phenyl groups.
The d-block metals form organometallic complexes with carbon or nitrogen donors such as porphyrin, 2-phenyl-pyridine, 2-thienylpyridine, benzo(h)quinoline, 2-phenylbenzoxazole, 2-phenylbenzothiazole or 2-pyridylthianaphthene and iminobenzenes. The (hetero)aromatic rings can be substituted for example as for the R and X groups given above. The emission of d-block complexes can be either ligand based or due to charge transfer. For the heavy d-block elements, strong spin-orbit coupling allows rapid intersystem crossing and emission from triplet states (phosphorescence).
In fluorescent electroluminescent devices, many excitons form in the non-emissive triplet state, reducing the efficiency of light emission.
Hence devices based on phosphorescent emitters, which can harvest the triplet excitons, have the potential for much higher efficiency than devices based on fluorescent emitters.
The dendrimers can be built in a convergent or divergent route, but a convergent route is preferred. Thus the dendrons are attached to the appropriate ligands and these are subsequently attached to the metal cation to form the dendritic metal complex. Optionally other non-dendritic ligands can subsequently be attached to said complex. Alternatively a ligand with a suitably reactive functional group can be complexed to the metal ion, and then reacted with appropriately functionalised dendrons. In this latter method, not all ligands have to have the reactive functional groups, and thus this method allows the attachment of dendrons to some but not all of the ligands complexed to the metal. A key property of the dendrons is to impart solution processibility to the metal complex and therefore allow the formation of good quality thin films suitable for use in light-emitting diodes.
The dendritic metal complexes may be homoleptic or contain more than one type of dendritic ligand, as discussed above. Alternatively, the metal complex may contain one or preferably more than one, e.g. 2 or 3, dendritic ligands plus one or more non-dendritic ligands. For example, with terbium complexes it is possible to have three dendritic ligands terminating in a carboxylate moiety for complexing to the metal plus one or more coligands to satisfy the co-ordination sphere of the metal cation. Suitable neutral co-ligands include 1,10-phenanthroline, bathophenanthroline, 2,2′-bipyridyl, benzophenones, pyridine N-oxide and derivatives of these. Also for iridium it is possible to have two dendritic phenylpyridine ligands with the third ligand a non-dendritic phenylpyridine ligand. It is desirable that the number of dendritic ligands is sufficient to provide the required solution processing. In the case of the dendritic metal complexes where all the ligands are different the method of preparation may give rise to a statistical mixture of all complex types. This is not necessarily disadvantageous providing that the optical, electronic, and processing properties are satisfactory. In the case of mixed dendron complexes it is preferable that the moieties forming the attachment point to metal are all the same or have similar binding constants. In the case of dendritic complexes that contain two or more different dendrons at least one should desirably be a conjugated dendron. The conjugated dendrons can be comprised of a number of different types of branching points.
The surface groups and dendrites can be varied so the dendrimers are soluble in solvents, such as toluene, THF, water and alcoholic solvents such as methanol, suitable for the solution processing technique of choice. Typically t-butyl and alkoxy groups have been used. In addition, the choice of dendron and/or surface group can allow the formation of blends with dendrimers (organic or organometallic), polymer or molecular compounds. In one embodiment of the present invention there is a blend of a phosphorescent dendrimer possessing an organometallic core and a dendrimer which possesses the same dendron type but a different core.
According to another aspect of the present invention the organometallic dendrimer can be incorporated into a light emitting device as either a homogeneous layer or as a blend with another dendrimer (organic or organometallic), polymer or molecular compound. In one embodiment we show the first example that we are aware of a d-block phosphorescent material being used as a homogenous light emitting layer in an LED. We have also found that when a phosphorescent organometallic dendrimer is blended with a fluorescent host the emission spectrum may depend on the driving frequency of electrical pulsing. A device can be driven by applying voltage (or current) pulses with a certain duration and period (together describing the driving frequency). Within the regime where the duration and/or period of the pulses are on a timescale of a similar order of magnitude to the phosphorescence decay lifetime then the emission spectrum may be sensitive to the driving frequency. In another embodiment, it has been found that it is advantageous to blend the dendrimer with a charge transporting material. In particular it has been found that the presence of a hole-transporting and/or a bipolar material and/or electron transporting material is advantageous. In a further embodiment the bipolar material should contain carbazole units. Another embodiment has one or more of each type of charge transporting material.
In one embodiment the device according to the invention comprises the dendrimer in the light emitting layer, preferably the dendrimer is the light emitting material. In a device according to the invention the dendrimer is preferably fluorescent in the solid state. In a device according to the invention the dendrimer is preferably phosphorescent in the solid state.
In a further embodiment in a device according to the invention the dendrimer is blended with at least one other dendrimer and/or polymer and/or molecular material. Preferably the organometallic dendrimer is phosphorescent in the solid state and is blended with a corresponding non-metallic dendrimer which possesses the same dendritic structure as that of the organometallic dendrimer, more preferably the molar ratio of organometallic dendrimer to other component is from 1: to 1:100. The device according to the invention may comprise, in addition to the light emitting layer, at least one charge transporting and/or injection layer. A further embodiment of the invention comprises a blend of an organometallic dendrimer according to the invention and a corresponding nonmetallic dendrimer having the same dendritic structure as that of the organometallic dendrimer, preferably the molar ratio or organometallic dendrimer to non-organometallic dendrimer is from 1:1 to 1:100
In a further embodiment in a device according to the invention the color of the emission is controlled by the duration and frequency of he driving electrical pulse.
The organometallic dendrimers can be incorporated into an LED in a conventional manner. In its simplest form, an organic light emitting or electroluminescent device can be formed from alight emitting layer sandwiched between two electrodes, at least one of which must be transparent to the emitted light. Such a device can have a conventional arrangement comprising a transparent substrate layer, a transparent electrode layer, a light emitting layer and aback electrode. For this purpose the standard materials can be used. Thus, typically, the transparent substrate layer is typically made of glass although other transparent materials such as PET, can be used.
The anode, which is generally transparent, is preferably made from indium tin oxide (ITO) although other similar materials including indium oxide/tin oxide, tin oxide/antimony, zinc oxide/aluminum, gold and platinum can also be used. Conducting polymers such as PANI (polyaniline) or PEDOT can also be used.
The cathode is normally made of a low work function metal or alloy such as Al, Ca, Mg, Li, or MgAl or optionally with an additional layer of LiF. As is well known, other layers may also be present, including a hole transporting material and/or an electron transporting material. When the dendrimer is a phosphorescent emitter, it has been found that it is particularly beneficial to have a hole-blocking/electron-transporting layer between the light emitting dendrimer layer and the cathode. In an alternative configuration, the substrate may be an opaque material such as silicon and the light is emitted through the opposing electrode.
An advantage of the present invention is that the layer containing the dendrimer can be deposited from solution. Conventional solution processing techniques such as spin coating, printing, and dip-coating can be used to deposit the dendrimer layer. In atypical device a solution containing the dendrimer is applied over the transparent electrode layer, the solvent evaporated, and then subsequent layers applied. The film thickness is typically 10 nm to 1000 nm, preferably less than 200 nm, more preferably 30-120 nm.